Most components used in building an oscillator such as a voltage controlled oscillator (VCO) have parameters that vary over temperature and input voltage. When the operating temperature of an oscillator changes, the transistor that comprises the amplifier section of the oscillator tends to have a corresponding change in gain. Changes in the input voltage of an oscillator circuit also affects the gain of the typical prior art oscillator circuit, thereby forcing the oscillator to draw more bias current.
A typical scheme used by the prior art for trying to maintain the gain of the transistor constant is by using constant current sources coupled to the transistor. The use of constant current sources help to maintain the current drawn by the active device substantially constant over its operating range. Constant current schemes unfortunately degrade the overall performance of the oscillator circuit and in particular the Hum and Noise figures of the communication device that the oscillator is part of. Also, constant current sources tend to be inherently noisy, thereby increasing the noise output of the oscillator circuit itself. Since the phase noise performance of an oscillator is very sensitive to changes in the transistor bias current of an oscillator, a need exists for the amplifier current to be maintained as constant as possible. For example, a shift of only 0.5 mA can cause an increase in phase noise of approximately a few dB's in a typical oscillator.
In FIG. 1 a schematic of a prior art voltage controlled oscillator (VCO) 100 is shown. Voltage controlled oscillator 100 is configured as a Hartley oscillator which is well known in the art. Oscillator circuit 100 includes an input port 102, oscillator section 108, output port 104, and operating voltage bias input port 106 (the required ground connections are shown inside of section 108). The oscillator section 108, includes an amplifier (active device) such as transistor 128. Transistor 128 is biased using resistors 110, 112, and 134. Voltage controlled oscillator 100 is typically used as part of a PLL synthesizer for use in communication equipment.
Typically, input port 102 receives a control voltage from a portion of a synthesizer like the one shown in FIG. 3. Oscillator 100 includes a tapped resonator element 124 having one end capacitively coupled via capacitor 126 to the base of NPN transistor 128, while the other end of the resonator is coupled to ground potential. Tapped resonator 124 is selectively tapped depending on the required operating parameters of oscillator 100 by one end of resistor 134 which is in turn coupled to the emitter of transistor 128. Tapped resonator 124 is used to provide the impedance transformation required to achieve the gain for oscillation. Oscillator 100 further includes a conventional tank circuit comprising of variable capacitor 122, capacitor 120, and varactor or "VARICAP" element 118. Oscillator 100 also includes capacitor 114 connected between input port 102 and ground, and a series connected inductor 116 between the input port and capacitor 120. The frequency of the Hartley oscillator 100 is varied by varying varactor 118 and variable capacitor 122. Also included as part of oscillator 100 is inductor 130 which is coupled between B+ terminal 106 and the collector of transistor 128. Finally, a coupling capacitor 132 is connected between the output port 104 and the collector of transistor transistor 128.
Since the operation of a conventional Hartley oscillator is well known to anyone skilled in the pertinent art, only the area of biasing of transistor 128 will be discussed. In the typical prior art oscillator, such as oscillator 100 of FIG. 1, the amplifier (in this case transistor 128) is normally biased by a series of resistors. In this particular case emitter resistor 134, and resistors 110 and 112 which form a voltage divider back to the base of transistor 128. The problem of biasing transistor 128 in this fashion is that a larger emitter resistance is typically required in order to properly bias transistor 128. Normally this is done to obtain the temperature stability and constant current characteristics across the frequency band of operation, which are required of a well designed oscillator. The problem with this approach is that the voltage across transistor 128 (V.sub.CE) drops since other components that cause other voltage drops are in series with transistor 128. This drop in voltage across transistor 128 causes a loss in noise performance and power for circuit 100.
As can be seen from the above discussion, a need exists for a more efficient way of stabilizing the amount of bias current drawn by an oscillator over changes in temperature and input voltage. A stabilizing scheme should also be able to be integrated in order for the oscillator circuit to take up less real estate, and be less costly to manufacture.